The present invention relates in general to a method and apparatus for magnetic recording/reproducing, and more particularly, to the digitization of audio signals for a video tape recorder (VTR) using a selected type of modulation and depth recording to avoid interference between the audio and video signals being recorded.
It is known to use a multi-layer recording method for recording video and audio signals in a VTR system. In this regard, attention is directed to the IEEE Transactions on Consumer Electronics, Vol. CE-30, No. 3, August 1984, pgs. 360-369, by Miura et al. As apparent from FIGS. 1 and 2 of this technical paper, a video signal is recorded on a surface part of the magnetic layer and the FM audio signal is recorded on a deep part thereof by separate video and audio heads, each of which has an azimuth angle different from the other. In the reproducing mode the video signal and the FM audio signal are separately reproduced with the help of the azimuth loss effect. However, both signals are recorded on the magnetic medium as analog signals in the Miura et al. system.
Various efforts have been made over the years to effect the recording of both of the video and audio signals on magnetic tape as digital signals. These efforts are directed to the possibility of video and audio signals on a magnetic tape comparable in size to the standard audio cassette. However, in order to superimpose the digital audio signal and the video signal, it is necessary to avoid interference between these signals, including interference resulting from the digital nature of the audio signal. In this regard, the spectrum of the digital audio base-band signal is such that, even if it is superimposed with the video signal by using the azimuth loss effect, it becomes impossible to separate the down-converted chrominance signal, if conventional base-band modulation of the digital audio signal is employed.
In the field of digital radio communication, a quadrature differential phase-shift keying (QDPSK) system is widely employed today due to some advantages including the capability of relatively narrow band transmission and the simplified structure of the demodulator. In such QDPSK system, however, passage of information through a non-linear channel causes an increase in the bandwidth and deteriorates the coding error rate characteristic. These disadvantages raise a serious problem to be solved in a transmitter for satellite communication, mobile communication or the like, where the bandwidth is limited and the carrier-to-noise power ratio (C/N) is not settable to be so high.
In view of the circumstances mentioned above, an offset QPSK system is attracting attention of late. This system is dealt with in the following: H. Suzuki et al., "Fundamental Properties of Narrowband Digital Angle Modulations", Shingaku Giho, CS-81-52, 1981; and R. K. Kwan, "The Effects of Filtering and Limiting a Double Binary PSK Signal", IEEE Trans. on AES, July 1986. According to this system, data of an in-phase (P) channel and a quadrature (Q) channel are modulated in such a manner that the code change points thereof have a mutual deviation of one half data period, wherein the envelopes of the modulated waves are rendered free from zero-crossing, and the band is not so widened with passage of the data via the non-linear channel. See also the paper by S. A. Rhodes, entitled "Effects of Hard Limiting on Band-Limited Transmissions with Conventional and Offset QPSK Modulation", IEEE National Telecommunication Conference, 1972.
However, it is impossible in such offset QPSK system to use differential coding and decoding employed customarily in the known QPSK system, so that detection of the absolute phase has been necessary on the demodulator side heretofore.